"It definitely had good teeth and was a very robust animal with a big brutish-looking snout," says Archer.

The one-metre species, which is about twice the size of the modern platypus is called Obdurodon tharalkooschild. Tharalkoo refers to a female duck in an Indigenous Australian Dreamtime story who is ravished by a water rat named Bigoon, resulting in a child that was a cross between a duck and a rodent - the first platypus.

The giant platypus was identified from a fossilised molar tooth discovered at Australia's famous Riversleigh World Heritage area.

"It's an extremely distinctive tooth," says Archer. "There's no other mammal group in the whole world that has teeth as strangely structured as platypuses."

"It's like these animals have looked at the teeth that placentals and marsupials have invented and put a mirror up to it."

Teeth and diet

The modern platypus lacks functional teeth and "bruises" its prey to death between "horny pads" in its mouth. The ancient species, on the other hand, would have happily crunched up small vertebrates including lungfish, frogs and small turtles.

Archer says the new species is unlikely to have been an immediate ancestor of the modern platypus. Instead, he says, it appears to be an "aberrant" side branch off.

"It's out of pace without the steadily-shrinking size of the other platypuses," says Archer. "It was a side branch exploring another option."

The researchers haven't yet been able to date the new fossil, but based on the evidence of animals found alongside it, they estimate that it is somewhere between 15 and 5 million years old.

Extinction risk

Archer says researchers "haven't a clue" why the platypus lost its teeth but this, together with its decrease in size and restriction in its distribution, suggests the animal is at risk of extinction.

"We've watched the evolution over the last 60 million years. They used to be very robust with fully functional teeth. They used to be in South America. They were in Antarctica. They were all over Australia," he says.

"Now of course they are smaller, they're kind of shrivelled up, they've lost their teeth and they're just in the eastern rivers of the continent."